In wireless cellular systems, transmit power control is desirable to minimize co-channel interference in order to maximize the number of active simultaneous traffic channels over an allocated frequency band, as discussed, for example, in K. S. Gilhousen, et al., “On the Capacity of Cellular CDMA Systems,” IEEE Trans. on Vehicular Technology, vol. 40, May 1991, which is incorporated herein by reference. Without appropriate power control, a code division multiple access (CDMA) system would not be able to achieve the capacity needed to serve the ever-demanding wireless markets today. The ITU has adopted both 3GPP (Third Generation Partnership Project) and 3GPP2 CDMA technology standards for the worldwide, third generation wireless cellular systems.
Several power control techniques are used in today's wireless cellular systems, including those described in T. Ojanpera and R. Prasad, Eds., “Wideband CDMA for Third Generation Mobile Communications”, Artech House Publishers, 1998; W. C. Y. Lee, “Mobile Cellular Telecommunications Systems”, McGraw-Hill, 1989; and D. Li and G. Yang, “Power-controlled Random Access,” U.S. Pat. No. 6,937,641, all of which are incorporated herein by reference. As is well-known by those skilled in the art, cellular systems typically use a high antenna tower (base station) to collect signals from mobile users and re-distribute signals in respective coverage areas. Also, one of the main goals of power control in cellular systems is to avoid the near-far effect.
In contrast to cellular systems, an ad-hoc wireless system is formed dynamically when a number of wireless nodes decide to join together to form a network. Each node communicates directly with many nodes rather than just one base station. Such a network uses peer-to-peer routing to form the network and route data messages across the network.
Because ad-hoc wireless networks operate substantially differently from cellular wireless systems, the power control techniques used or proposed for cellular systems are not suitable in an ad-hoc environment.
Power control and data rate control techniques for ad-hoc wireless networks have conventionally concentrated on the reservation channel by measuring the signal-to-noise ratio (SNR) of the received request to send (RTS) packet (including its variants: hello RTS (HRTS), broadcast RTS (BRTS), and packet RTS (PRTS)) in order to adjust the transmit power level and data rate on the message channel. The transmit power on the message channel may also be adjusted by the acknowledge (ACK) completion rate. Typically, when the ACK completion rate drops below a pre-configured threshold, the transmit power is increased first. If the power adjustment does not improve the ACK completion rate, the transmission data rate is then lowered. These adjustments are targeted at both improving the message completion rate and minimizing the battery power drain.
The basic ad hoc wireless network power control and data rate adjustment approach mentioned above is based on a theoretical symmetrical radio link. That is, it is assumed that the receive sensitivity on both the transmitting node and the receiving node is the same. However, the receive sensitivity can vary substantially among individual radios, sometimes by as much as 6 dB. In practice, ensuring a symmetrical link in an adhoc wireless system is, for all practical purposes, economically unfeasible in that the electronic radio components that would be needed to keep the tolerances to a very minimum level across a wide range of operating conditions, such as temperature, humidity, etc. would be too costly. In any event, there is no particular reason to have a symmetrical link design. Radios can operate just as well on an asymmetrical link.
As noted, the basic power and data rate adjustment technique discussed above is based solely on the measured SNR on the reservation channel. It has been determined that this may lead to an incorrect power level and data rate setting when the sender has a better receive sensitivity than the receiver.
When the sender assumes that the receiving node has the same receive sensitivity and sends the data packet based on its own measured SNR, the receiving node will not be able to demodulate the data correctly if its receive sensitivity is worse than the sender's. Conventional transmit techniques repeat transmissions at the same data rate and power level until the next evaluation period. Consequently, during a given control period, the sender unnecessarily wastes battery power and keeps the access channel busy with multiple failed access attempts by repeatedly sending multiple RTS-Data sequences, without receiving a corresponding ACK or NACK in return.
There have been several attempts at power control in ad-hoc wireless networks. For example, T. J. Kwon and M. Gerla, “Clustering with Power Control”, IEEE MILCOM, vol. 2, pp 1424-1428, November 1999, describes an attempt to impose a cellular structure to an ad-hoc network topology. Each cluster head acts like a cellular base station. The authors propose to use the power control algorithms in a similar fashion, as used in cellular systems, to control the size of a cluster. This approach imposes a centralized resource distribution onto an ad-hoc wireless system. However, because an ad-hoc wireless system is distributed in nature, a distributed scheme would be a more efficient and flexible approach.
R. Ramanathan and R. Rosales-Hain, “Topology Control of Multihop Wireless Networks Using Transmit Power Adjustment”, IEEE INFOCOM, vol. 2, pp. 404-413, March 2000, describes a similar approach, but formulates the problem differently, namely as an optimization problem. The authors determine the optimal number of neighbors that each node should have to minimize the maximum transmit power while maintaining connectivity constraint. In the technique that is proposed, the number of neighbors of a node is restricted by reducing the node's transmit power by a constant value. Such a restriction on the number of nodes inhibits the flexibility of the network.
J. Haartsen, “System and Method for Link Adaptation in Communication Systems,” U.S. Pat. No. 6,944,460; J. M. Belcea, “System and Method for Providing Adaptive Control of Transmit Power and Data Rate in an Ad-Hoc Communications Network,” U.S. Pat. No. 6,904,021; and R. M. D'Souza, S. Ramanathan, and D. T. Lang, “Adaptive Power Level Setting in an Ad-Hoc Wireless Network,” U.S. Pat. No. 6,970,714, power-up and power-down control and data rate adjustment technique. However, the disclosed techniques that control the transmit power and data rate apply only to single-channel networking systems. These techniques do not address a multi-channel scheme that is required to achieve the high-capacity systems that are in demand today.
Thus, while previous proposals and implementations for transmit power control and data rate adjustment have addressed the cellular as well as relatively more harsh and dynamic ad-hoc networking environments, there remains a need for more effective power control techniques or methods for capacity-hungry mobile networks, especially ad-hoc networking systems.